Tremolo producing arrangement in which the tremolo signal is isolated from the audio circuit



Jan. 13, 1970 E. J. MEIXNER 3,490,059

TREMOLO PRODUCING ARRANGEMENT IN WHICH THE TREMOIJO SIGNAL IS ISOLATEDFROM THE AUDIO CIRCUIT Filed June 28, 1966 67511040 .S/GA/AL 5 nvvewromUnited States Patent U.S. Cl. 841.24 9 Claims ABSTRACT OF THE DISCLOSUREA tremolo variation is impressed upon an electrical tone signal, withoutintroducing the tremolo signal into the tone signal transmissionchannel, by utilizing a circuit that provides a variable impedance shuntpath for the tone signal. A pair of back-biased series diodes isconnected in parallel with a transistor which is the central componentof a -DC voltage divider. Application of the tremolo signal to thetransistor varies the bias of the diodes, and hence the impedance of thediodes with respect to the tone signal conveyed to the juncture point ofthe diodes, the juncture point being maintained at a constant voltage bythe action of the voltage divider.

This invention generally relates to an arrangement for producing atremolo effect in a musical signal, and more particularly this inventionrelates to a tremolo producing arrangement for an electrical musicaltone signal in which the tremolo signal is isolated from the tone signalchannel.

It has long been recognized that a cyclical variation in the amplitudeof a musical tone at a low frequency creates a pleasurable listeningeffect. This effect is generally known as a tremolo. Prior artarrangements for producing a tremolo effect in electrical musicalinstruments may generally be broken into two basic approaches: (1)modulation of tone signal amplitude by control of the tone generator oran amplifier; and (2) variable attenuation of a tone signal by using avariable impedance device in the tone signal transmission channel. Bothof these approaches have the disadvantage that the tremolo signal isintroduced into the tone signal transmission channel. Introduction ofthe tremolo signal into the tone signal transmission channel can producedistortion and other undesired effects. For instance, if the modulatingtremolo signal is not sinusoidal, a thump may be produced in the audiooutput.

A third approach that may be utilized in producing a tremolo effect isto provide a circuit connected from the tone signal transmission channelto ground. This circuit has a variable impedance so that the tone signalmay be variably bled-off or attenuated. Prior art arrangements thatutilize this approach have the same disadvantage as the twoaforementioned basic approaches, i.e., that the tremolo signal isintroduced into the tone signal transmission channel. This is truebecause the variable impedance to ground is usually directly controlledby the tremolo signal, so that the voltage level in the tone signaltransmission channel is repetitively altered to variably attenuate thetone signal at a tremolo rate. The variation of the voltage level in thetone signal transmission channel results in the tremolo sign-a1 actuallybeing introduced into the tone signal transmission channel. Thisproduces a superposition of the tremolo signal upon the tone signal.

To eliminate the undesirable introduction of the tremolo signal into thetone signal transmission channel while producing a tremolo, the presentinvention was evolved. Briefly, in the preferred embodiment disclosedherein, the subject invention involves a shunt or Need Patented Jan. 13,1970 0E path including a tremolo producing circuit connected to the tonesignal transmission channel to produce a tremolo effect. In order toproduce a tremolo without introducing the tremolo signal into the tonesignal transmission channel, the shunt path has a variable impedance forthe tone signal, but a constant voltage level is maintained at the pointin the tremolo producing circuit to which the tone signal transmissionchannel is connected. This is achieved by utilizing a tremolo producingcircuit that is balanced about a constant voltage point in the circuit.

For example, in the preferred embodiment disclosed herein, a pair ofdiode-s are connected in series, with the common juncture of the diodes(the constant voltage point of the circuit) being connected to the tonesignal transmission channel through a blocking capacitor. The seriesconnected diodes are connected in series with and placed between a pairof resistors having equal magnitudes. One of the resistors is connectedto a DC voltage source, while the other is connected to ground. Theseries connected diodes are arranged to be back-biased by the DCvoltage. A transistor is positioned in the circuit with itsemitter-collector circuit in parallel with the series diodes, i.e., thecollector is connected to one side of the series diode arrangement,while the emitter is connected to the other side of the series diodearrangement. The base of the transistor is adapted to receive a tremolosignal.

Since the resistors and diodes are balanced, the voltage level at thecommon juncture of the diodes (constant voltage point) is always halfthe magnitude of the applied DC voltage, regardless of the conductingstate of the transistor. However, the conducting state of the transistorwill determine the voltage appearing across the-series connected diodes,thereby setting the bias state of the diodes. As the bias state of theseries connected diodes is altered, the impedance of the circuit to thetone signal varies at a rate determined by the frequency of the tremolosignal applied to the base of the transistor. Thus, the amount of thetone signal that is shunted through the blocking capacitor and thetremolo producing circuit varies at a tremolo rate to variably attenuatethe tone signal at this rate and to produce a tremolo effect in theaudio output. In this manner, a tremolo effect is produced, and thebalanced circuit arrangement prevents a change in the voltage level inthe tone signal channel, so that the tremolo signal is not directlyintroduced into the tone signal transmission channel. Therefore, themagnitude of the tone signal is variably attenuated as the signalalternates about a fixed potential level, rather than having :a tremolosignal superposed thereon.

Accordingly, a primary object of this invention is to provide a tremoloproducing arrangement which does not introduce the tremolo signaldirectly into the tone signal transmission channel.

Another object of this invention is to provide a tremolo producingarrangement which does not cause distortion or other undesirable effectsin the ultimate audio output.

Yet another object of the present invenion is to provide a tremoloproducing arrangement which is relatively easy to construct and whichutilizes solid state circuitry.

These and other objects, advantages, and features of the subjectinvention will hereinafter appear, and, for purposes of illustration,but not of limitation, an exemplary.

embodiment of the subject invention is shown in the appended drawing,the single figure of which illustrates a schematic circuit diagram of apreferred embodiment of the present invention.

In the circuit diagram of the sole figure of the drawing, a tone signalline or transmission channel 1 is schematically illustrated. A seriesimpedance 3 is located in tone signal transmission channel 1.

A tremolo producing arrangement or circuit 5 is operatively associatedwith tone signal transmission channel 1. Tremolo producing arrangement 5includes a pair of series connected diodes 7 and 9. Diode 7 has a plate11 and a cathode 13, while diode 9 has a plate and a cathode 17. Diodes7 and 9 are selenium diodes which have a linear portion of theircharacteristic curve passing through the no-bias point. The commonjuncture of diodes 7 and 9, where plate 11 of diode 7 is connected tocathode 17 of diode 9, defines a constant voltage point 18.

A resistor 19 is connected to cathode 13 of diode 7, and a resistor 21is connected to plate 15 of diode 9. Resistors 19 and 21 are matching(i.e., identical), as are diodes 7 and 9.

A transistor 23 is connected across the series connected diodes 7 and 9.As illustrated, transistor 23 is of the NPN type and has an emitter 25,a base 27, and a collector 29. Collector 29 of transistor 23 isconnected to the junction of resistor 19 and cathode 13 of diode 7, andemitter of transistor 23 is connected to the junction of resistor 21 andplate 15 of diode 9, so that the emittercollector circuit of transistor23 is in parallel with series connected diodes 7 and 9.

A DC voltage source 31, indicated schematically as a conventionalstorage battery, provides a bias voltage for the circuit. The positiveside 33 of source 31 is connected to resistor 19, while the negativeside 35 of source 31 is connected to ground, as is resistor 21. Acapacitor 36 connected to ground from the positive side 33 of source 31indicates that this side of the source is at ground potential for an ACsignal. These connections of DC voltage source 31 are such that diodes 7and 9 are backbiased, but within the linear portion of thecharacteristic curve. The balanced arrangement of resistors 19, 21 anddiodes 7, 9 causes the applied DC potential to be apportioned in such amanner that point 18 is always at a constant voltage level equal toexactly half the magnitude of the potential supplied by DC voltagesource 31. Point 18 (which as previously indicated is located at thecommon juncture of diodes 7 and 9) is the point at which tremoloproducing arrangement 5 is connected to tone signal channel 1 through ablocking capacitor 39. Blocking capacitor 39 passes the tone signal totremolo producing circuit 5, but prevents DC voltage in circuit 5 frombeing applied directly to tone signal transmission channel 1.

A bias arrangement for base 27 of transistor 23 is provided by seriesresistors 41 and 43 connected across DC voltage source 31. Resistor 41is connected between positive side 33 of source 31 and a point 45, andresistor 43 is connected between grounded negative side 35 of source 31and point 45. A tremolo signal is applied to the circuit at point 45from a conventional tremolo signal source 46. Tremolo signal source 46is any conventional generator of low frequency signals used in tremoloor vibrato producing circuits. A tremolo signal appearing at point 45 isconnected to base 27 of transistor 23 to vary the conductivity oftransistor 23 at the tremolo rate.

A tone signal appearing on tone signal transmission channel 1 has afrequency such that it will be transformed into a specified audio toneby a suitable converter (not shown), such as a conventional loudspeaker.To produce a tremolo effect, the tone signal is variably attenuated at atremolo rate. To achieve this effect, the tone signal is connected topoint 18 of tremolo producing arrangement 5 through blocking capacitor39.

Due to the balanced arrangement of diodes 7 and 9 and resistors 19 and21, the voltage level at point 18 has a fixed value, i.e., one-half themagnitude of the.

applied potential supplied by DC voltage source 31. This is true inspite of any variations that occur in the conductivity state oftransistor 23. For example, if transistor 23 is transferred to a lowerconductivity state by the signal applied to base 27, the voltage dropacross the emiter-collector circuit of transistor 23 will increase. Theincrease in the voltage drop across transistor 23 means that the voltagedrop across resistors 19 and 21 must be decreased. However, sinceresistors 19 and 21 are equal, the voltage drops across these resistorswill still have the same value, although this value is less than it wasbefore the decrease in conductivity of transistor 23. At the same time,the increase in the potential across transistor 23 increases the voltagedrop across the series connected diodes 7 and 9. Since the voltage dropacross the complete circuit remains constant at the magnitude of thepotential supplied by DC voltage source 31, the decreased voltage dropacross resistor 19 (for example) must be exactly equaled by the increasein voltage drop across diode 7. This is true because diodes 7 and 9 arematched so that the voltage drops across these diodes are equal (as isalso true of resistors 19 and 21), although the magnitude of the voltagedrops vary with changes in the conductivity state of transistor 23.Therefore, the voltage level at point 18 remains constant throughoutchanges in the conductivity state of transistor 23.

As pointed out above, although the potential at point 18 remainsconstant, the potential across diodes 7 and 9 will vary with changes inthe conductivity state of transistor 23. Thus, the biasing of diodes 7and 9 varies with changes in the conductivity state of transistor 23.For example, if the voltage across the emitter-collector circuit oftransistor 23 is v volts, the voltage drop across each of the diodes 7and 9 will be v/2. This sets the operating characteristics of the diodesat a specified point on the linear portion of the characteristic curvecorresponding to a bias of v/2. Positive portions of the tone signalwill be bled to AC ground (as symbolized by capacitor 36) through diode7 and negative portions of the tone signal will be bled to groundthrough diode 9. At the chosen operating point diodes 7 and 9 will havea predetermined impedance to the respective portions of the tone signal.As the operating point is varied up and down the linear portion of thecharacteristic curve, the impedances will vary correspondingly. Due tooperating on the linear portion of the characteristic curve, the tonesignal is not distorted by the variable impedance path.

By varying the conductivity of transistor 23 at a tremolo rate, theattenuation of the tone signal in the tone signal transmission channel 1may be varied at a tremolo rate to produce a tremolo effect in the audiooutput. However, the balanced arrangement maintains a constant potential at point 18 so that the tremolo signal applied to transistor 23 isnot directly introduced into tone signal transmission channel 1. In thisfashion, the tremolo effect is produced without incurring the problemsthat are encountered when the tremolo signal is introduced into the tonesignal transmission channel 1.

It should be understood that the embodiment described is merelyexemplary of the preferred practices of the present invention and thatvarious changes, modifications, and variations may be made in thearrangements, operations, and details of construction of the elementsdisclosed herein, without departing from the spirit and scope of thepresent invention, as defined in the appended claims.

What is claimed is:

1. A tremolo producing arrangement for an electrical tone signalappearing on a tone signal transmission channel comprising:

a source of tremolo signal;

variable impedance circuit means located outside the tone signaltransmission channel to provide a shunt impedance for the electricaltone signal, said shunt impedance being varied in response to saidtremolo signal;

control means in said variable impedance circuit for maintaining a pointin said variable impedance circuit at a constant voltage independentlyof circuit im. pedance changes therein; and

coupling means for connecting said constant voltage point to the tonesignal transmission channel,

whereby the electrical tone signal may be variably attenuated at atremolo rate without introducing said tremolo signal directly into thetone signal transmission channel.

2. An arrangement as claimed in claim 1, wherein said control meanscomprises:

a DC voltage source;

a pair of unidirectional conducting devices connected in series, withsaid constant voltage point located at the common juncture of saidseries connected unidirectional conducting devices;

tremolo signal controlled means connected in parallel with said seriesconnected unidirectional conducting devices to vary the impedancethereof at a tremolo rate; and

a pair of matched resistors connected in series with and on oppositesides of said tremolo signal controlled means, said DC voltage beingapplied across the series combination comprising said tremolo signalcontrolled means and said pair of matched resistors.

3. An arrangement as claimed in claim 2, wherein said series connectedunidirectional conducting devices are matched solid state diodesarranged to be back-biased by said DC voltage source.

4. An arrangement as claimed in claim 2, wherein said tremolo signalcontrolled means comprises a transistor having base, emitter andcollector electrodes.

5. An arrangement as claimed in claim 4 wherein:

said source of tremolo signal is connected to said base electrode ofsaid transistor;

said series connected unidirectional conducting devices are matchedsolid state diodes having plate and cathode terminals and are connectedin series between said emitter and said collector electrodes of saidtransistor, with the cathode of one of said diodes being connected tothe plate of the other of said diodes at said constant voltage point,the cathode of the other of said diodes being connected to saidcollector electrode of said transistor, and the plate of said one ofsaid diodes being connected to said emitter electrode of saidtransistor;

the positive side of said DC voltage source is applied to the one ofsaid pair of matched resistors connected to said collector electrode ofsaid transistor; and

said coupling means comprises a capacitor connected between saidconstant voltage point and said tone signal transmission channel.

6. An arrangement as claimed in claim 1, wherein said control meanscomprises:

a DC voltage source;

a voltage divider including a variable impedance element the impedanceof which varies in response to said tremolo signal, said DC voltagesource being connected across said voltage divider; and

a variable impedance device connected in parallel with said variableimpedance element and including said constant voltage point.

7. An arrangement as claimed in claim 6 wherein said variable impedanceelement is a transistor having said tremolo signal applied to the baseelectrode thereof.

8. An arrangement as claimed in claim 7 wherein said voltage dividerfurther comprises matched resistors, one on either side of saidtransistor.

9. An arrangement as claimed in claim 6 wherein said variable impedancedevice comprises a pair of diodes connected in series, said constantvoltage point being located at the juncture of said diodes.

References Cited UNITED STATES PATENTS HERMANN KARL SAALBACH, PrimaryExaminer SAXFIELD CHATMON, JR., Assistant Examiner US. Cl. X.R.

